Fix stale copy, dedupe Exit columns, document local report review

Follow-ups from review of the Track Record slim:

- BacktestPanel: drop the stale "tracking check" sentence from the "How this is
  measured" explainer — that check moved to the maintenance disclosure last
  commit, so it no longer describes anything in this block.
- MyTradesPanel: rename the two identically-labelled "Exit" columns to "Exit Px"
  (exit price) and "Reason" (close_reason) so they're not confusable.
- README: add "Reading a local backtest report" under Local Backtest Snapshots —
  a section->decision map for reports/backtest-*.json. The strategy-tuning tables
  removed from the deployed page (sweep, gate_ablation, time_exit_sweep,
  signal_eval, strategy_variants) now live only in the local report, so this
  keeps "research lives local" from meaning the decision knowledge evaporates.

tsc -b && vite build pass.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/components/signals/MyTradesPanel.tsx

@@ -93,11 +93,11 @@ export function MyTradesPanel() {
                   <th className="px-4 py-2.5">Ticker</th>
                   <th className="px-4 py-2.5">Dir</th>
                   <th className="px-4 py-2.5 text-right">Entry</th>
-                  <th className="px-4 py-2.5 text-right">Exit</th>
+                  <th className="px-4 py-2.5 text-right">Exit Px</th>
                   <th className="px-4 py-2.5 text-right">P&L</th>
                   <th className="px-4 py-2.5 text-right">R</th>
                   <th className="px-4 py-2.5 text-right">Alpha</th>
-                  <th className="px-4 py-2.5">Exit</th>
+                  <th className="px-4 py-2.5">Reason</th>
                   <th className="px-4 py-2.5 text-right">Closed</th>
                 </tr>
               </thead>